1. FIELD OF THE INVENTION
The present invention relates to .alpha.-aspartyl-phenylalanine ester represented by the general formula (I) ##STR3## wherein R' represents an alkyl group having 1 to 4 carbon atoms.
.alpha.-Aspartyl-phenylalanine ester is useful as a low calorie sweetening agent, and have attracted public attention recently.
2. DESCRIPTION OF THE PRIOR ART
As a process for producing .alpha.-aspartyl-phenylalanine ester, there has already been proposed a process which comprises condensing .alpha.-[3-substituted-5-oxooxazolidinyl-(4)]acetic acid represented by the general formula (II) ##STR4## wherein R represents an organic moiety which can be substituted reductively by hydrogen, with phenylalanine alkyl ester represented by the general formula (III) ##STR5## (wherein R' is as defined above) to obtain an N-protected-.alpha.-aspartyl-phenylalanine ester represented by the general formula (V) ##STR6## (wherein R and R' are as defined above) and reducing the compound (V) in a hydrogen atmosphere to obtain .alpha.-aspartyl-phenylalanine ester represented by the general formula (I) (Japanese Patent Publication No. 812/73). However, according to detailed study of the above-mentioned process made by the present inventors, it was found that the condensation of the compound (II) with the compound (III) does not give the compound (V), but give an N-protected-N-hydroxymethyl-.alpha.-aspartyl-phenylalanine ester represented by the general formula (IV) mentioned later, and that during reduction step, a hydroxymethyl group of the compound of the general formula (IV) is reduced to form a methyl group to give N-methyl-.alpha.-aspartyl-phenylalanine methyl ester (hereinafter referred to as "N-methyl form") as by-product, so that the yield of the compound of the general formula (I) is low.
On the other hand, there has also been proposed a process for producing a compound of the general formula (I) which comprises reducing an N-protected-.alpha.-aspartyl-phenylalanine ester represented by the general formula (V) ##STR7## wherein R and R' are as defined above, with formic acid in the presence of a palladium catalyst (EP 99855).